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This question already has an answer here:

According to genome.gov, All the cells in a person's body have the same DNA.

I understand that the main reason parents and children have different genomes is because of random mixing of 2 genomes, but that there are also very small differences caused by random mutations.

If all of a person's cells have identical genomes, then does that mean that random mutations only occur in gametes?

I also understand that if sunlight hits my arm, it can cause random mutations.

So do all cells in a person's body really have the same DNA?

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marked as duplicate by Jack Aidley, David, AliceD Sep 26 '18 at 19:08

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All cells in a person’s body have roughly the same DNA. As you point out correctly, mutations can happen in any cell at any time. They are frequently caused by

  • UV radiation
  • Chemicals (think of the correlation between the development of lung cancer and smoking)
  • Replication of DNA during cell division
  • Integration of foreign (e. g. viral) genomic material
  • On purpose by the body’s own enzymes

That last point brings us to cells of the immune system, some of which (classically T- and B- Lymphocytes, but there is emerging evidence for others) generate variance within their genomic DNA on purpose.

Red blood cells lack a nucleus altogether.

Gametes only have half the body’s genome, as you already pointed out.

Finally, although having the same base sequence, epigenetic modification of DNA molecules by, for example, Methylation makes DNA chemically different even between cells of the same type.

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Mutations do also happen during mitosis, hence leading to cell lineages in your body to differ genetically by some mutations. While the vast majority of these mutations have no significant effec ton the individual health, some of these mutations may lead to serious health issues such as cancer. Also, you might want to consider that some cells are anucleated (red blood cells typically) and hence do not contain any nuclear DNA.

So, in short, the statement

All the cells in a person's body have the same DNA

is a "good approximation" but is not "exactly correct"!

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